1. Field of the Invention
This invention relates generally to geologic surveys and more particularly to an apparatus and method for acquiring and processing seismic data.
2. Description of the Related Art
Conventional geophone and hydrophone systems used in seismic prospecting typically have several sensors that produce analog signals indicative of a seismic wave. The seismic wave is usually produced by an energy source such as a vibrator truck, explosives or by an air gun in the case of a hydrophone system. These seismic signals are then conducted to acquisition/conversion circuitry. The analog signals from one or more remote seismic sensors (hydrophones, geophones, or other seismic sensors) are sampled and converted to a series of digital values by the acquisition/conversion circuitry. The acquisition/conversion circuitry is typically configurable to, for example, adjust the sampling rate, alter any digital filtering or other digital signal processing parameters, or perform diagnostics.
One or more of these acquisition/conversion circuits are connected to a data collection unit. Each data collection unit collects the series of digital values for all the seismic sensors connected to all the acquisition/conversion units connected to it. The data collection unit passes that data to a system controller, usually the truck, which includes a seismic recording device or Central Recording System (“CRS”).
A conventional system as described above is typically used in the seismic industry to enable a seismic data acquisition method called remote digitization. In this method a small number of analog signals are conveyed by wire to an analog to digital converter called a “field box” located remotely from the Central Recording System. In this field box analog signals acquired by the sensors are converted to digital form. Immediately after the conversion, digital data are transmitted to the CRS via serial communication. Typically, a processor and software are used to assign a time slot for transmitting the data. By example, the box closest to the CRS is assigned the first time slot and the next box the second time slot and so forth. A set of digital values from a field box associated with a particular time slot is called a trace. After all of the signals are digitized synchronously, each field box transmits the first trace at the first time. Then the second box would transmit the data for the first trace in the second time slot and so forth down the line. After all of the trace data for the first time slot are transmitted, i.e. time-one samples, then the process is repeated for another trace from all of the boxes i.e. time-two samples. In this manner, all of the data from the remote field units is transmitted to the CRS.
Early in the development of remote digitization systems the data were immediately written to tape with all of time-one samples from all of the traces followed by time-two samples of all the traces. This method is called multiplexed. In larger systems, the CRS typically uses the known structure of the data to collect all of the time samples for one location or trace in sequential memory or tape location. This organization is called demultiplexed and is needed by the processing systems that will receive the seismic data.
The conventional system has several limitations, especially as the number of traces in the recording system increase or redundant methods are needed to improve the reliability of the system. The order that data arrives at the CRS is used to imply or calculate the location of the field box sending the data to the CRS. Using arrival timing in this fashion means that the data cannot be sent via a route other than the predetermined initial route. If traces are contaminated during, transmission they must continue to be passed through the system to preserve the location so that the CRS can keep track of location. This contaminated data causes unexpected errors and failures of the system. The system must add some bits to the data that is transmitted to control the transmission. Because each data value is sent by itself, immediately upon acquisition, the overhead becomes very large and limits the amount of seismic data that can be transmitted over a single channel.
Another drawback of the conventional system is the time required to recover from corrupted or otherwise unusable data packages transmitted from the data collection units to the main controller recorder.
Another drawback of the conventional system is that a system designer typically must decide to use fiber optic cable or wire conductor cable to interconnect components regardless of system length requirements. The typical system component having fiber optic connectors is prone to failure caused by environmental conditions and is costly to use for shorter system lengths. Although copper wire is cost effective at shorter distances, a wire cable has a limited frequency response over longer distances and is much more cumbersome to deploy and retrieve.